The gravitropic response of roots involves perception of the 'gravistimulus' by root-cap cells, transduction of this physical information into biochemical signals, followed by their transmission to cells in the expansion zone. This ultimately results in an appropriate curvature response of the root. Indole-3-acetic acid (IAA), the major form of auxins in higher plants, is directly involved in providing directionality to plants in response to stimuli such as light, gravity and even obstacles in the path of growth but its mechanism of action has not been elucidated. Plants bearing mutant alleles of EIR1, which encodes a plasma membrane protein, have phenotypes that suggest it is an auxin efflux pump involved in transport of auxin from the root tip to the elongation zone. These mutants have long roots with compromised ability to form root hairs and the roots do not respond to light or gravity. These mutants resemble plants grown in the presence of auxin transport inhibitors. The focus of this proposal is: 1) to further characterize Eir1p, a key component involved in the gravitropic response of roots; and 2) to identify proteins that affect activity of the EIR1 pump. These studies will provide insight regarding the previously uncharacterized auxin signaling pathway involved in the gravitronic response.